Epidermal remodeling in psoriasis (II): a quantitative analysis of the epidermal architecture

Hyperproliferative psoriatic epidermis was quantitatively analyzed using a geometric model of viable epidermis. Our model was based on hexagonally arranged cylindrical papillae, which allowed the determination of the total volume of the viable epidermis and the total area of the interface with the proliferative compartment based on several parameters, such as papillary height, papillary width, and distance between neighboring papillae. The analysis assumed that the total number of viable epidermal cells paralleled the proliferative compartment in a steady state of cell flow, so a quantitative relation could be made between both volume and interface of the viable epidermis. Multiple parameters of the psoriatic epidermal architecture were measured, and variations within psoriasis were predicted by the model. The results predicted were remarkably close to the observed values. The geometric model also indicated that psoriatic epidermis could be subdivided into two distinct types, with and without a granular layer; the latter having a shorter turnover time. This is consistent with the notion that the typical psoriatic epidermis (without the granular layer) represents the expanding hyperproliferative phase, whereas the psoriatic epidermis with a granular layer represents stationary or resolving states. The model of hexagonally arranged cylindrical papillae suggested that the architecture of the psoriatic epidermis is constructed by a simple mechanism, whereby the psoriatic angulated rete-papilla pattern was produced by a two-dimensional increase in the proliferative compartment and a three-dimensional increase in the total volume of the viable epidermis.